Tanks don't usually leave for no reason, they do however leave for the following reasons:
DPS/Healer is not prepared or properly geared (ilvl system is still a joke, no one in pieces of af gear should be queuing for HK HM or any of the tougher dungeons)
DPS/Healer does not know the fights and as selfish individuals expect to be carried despite having a pletheora of both free and easy to access sources that will teach them what they need to know quickly
DPS/Healer disregard fight mechanics or don't bother dodging them and as a result make the party suffer and wipe
DPS/Healer is poor at playing their class because they have not taken the time to learn it
Just as we are expected to be good tanks, know our class, the positions, and the fight, it is equally expected of all players and in all roles to do the same. One thing that enrages me a lot about the younger generation of players that are also present in this game, is they believe they can join an instance, any fight, without having first prepared themselves properly be it through their gear or knowledge of the fight. As I've said, there are too many resources available to give any logical explanation as to why you do not know the fight despite it being your first time, nor your class. Any objection to the contrary is selfish, shortsighted, and arbitrary.
You want us to stay in groups? Run the proper instances multiple times to get within a decent ilvl range before you queue for harder things. I have a Blm/Smn/Sch/Monk/Warrior i70lvl or higher, and I geared each by starting and farming the appropriate ilvl dungeons (CM runs, lots of CM runs) before I attempted harder things because I wanted to carry my weight and provide an enjoyable experience for my other team mates while not wasting everyone's time or dragging out the instance.
Its a mark of common courtesy and speaks volumes about the type of player you are; it decides if you're simply a selfish scrub or a skillful participant.
TL : DR they left because your party was bad.